Field of the Invention
The exemplary, illustrative, technology herein relates to Solid Oxide Fuel Cell (SOFC) systems, methods of use, and methods of manufacturing SOFC systems. In particular, the exemplary, illustrative technology relates to improved systems and methods for thermal energy management within the SOFC system.
The Related Art
A conventional SOFC system includes a hot zone, which contains or at least partially encloses system components that are maintained at higher operating temperatures, e.g. above 350 or 500° C., during operation, depending on the SOFC technology. The hot zone houses an SOFC energy generator or solid oxide fuel cell stack. Conventional SOFC fuel cell stacks are formed by one or more fuel cells with each cell participating in an electro-chemical reaction that generates an electrical current. The fuel cells are electrically interconnected in series or in parallel as needed to provide a desired output voltage of the cell stack. Each fuel cell includes three primary layers, an anode layer or fuel electrode, a cathode layer or air electrode and an electrolyte layer that separates the anode layer from the cathode layer.
The anode layer is exposed to a gaseous or vaporous fuel that at least contains hydrogen gas (H2) and/or carbon monoxide (CO). At the same time the cathode layer is exposed to a cathode gas such as air or any other gas or vaporous oxygen (O2) source. In the cathode layer oxygen (air) supplied to the cathode layer receives electrons to become oxygen ions (O+). The oxygen ions pass from the cathode layer to the anode layer through the ceramic electrolyte layer. At the triple phase boundary, in the anode layer, hydrogen (H2) and/or carbon monoxide (CO) supplied to the anode layer by the fuel react with oxide ions to produce water and carbon dioxide and electrons emitted during this reaction produce electricity and heat. Other reaction by products in the fuel stream may include methane, ethane or ethylene. The electricity produced by the electro-chemical reaction is extracted to DC power terminals to power an electrical load.
Common anode materials include cermets such as nickel and doped zirconia (Ni-YSZ), nickel and doped ceria (Ni-SDC and or Ni-GDC), copper and doped ceria. Perovskite anode materials such as (La1-xSrx)Cr1-yMyO3-δ (LSCM) and other ABO3 structures are also usable. Common cathode materials include Lanthanum Strontium Cobalt Oxide (LSC), Lanthanum Strontium Cobalt Iron Oxide LSCF and Lanthanum Strontium Manganite (LSM). The electrolyte layer is an ion conducting ceramic, usually an oxygen ion conductor such as yttria doped zirconia or gadolinium doped ceria. Alterably the electrolyte layer is a proton conducting ceramic such as barium cerates or barium ziconates. The electrolyte layer acts as a near hermetic barrier to prevent the fuel and air from mixing and combusting.
Conventional SOFC systems use cross flow or parallel flow heat exchangers, commonly referred to as recuperators, to heat cathode gasses (air) entering the SOFC system. The gas flow heat exchangers heat cool air entering the hot zone exchanging thermal energy between the cool entering air and hot exhaust gas exiting the hot zone.
It is known to include one or more thermal energy or heat sources disposed inside the SOFC hot zone to heat the air and fuel flowing through the SOFC system and to heat the fuel cells. The heat source may include a tail gas combustor used to combust spent fuel mixed with hot exhaust air as the spent fuel and exhaust air exit the cell stack. A second heat source may include a cold start combustor operable to combust fuel at system startup to heat the SOFC surfaces and to heat incoming fuel flowing to the cell stack at least until the SOFC systems reaches it steady state operating temperature or the CPDX or TGC lights off. Electrical heating elements are also usable instead of or in addition to a cold start combustor to heat air, fuel, and operating surfaces at startup.
In conventional SOFC systems thermal energy is primarily transferred by gas to gas or gas to surrounding surface thermal energy exchange, i.e. primarily by convection. This occurs in the tail gas combustor when spent fuel is mixed with hot exhaust air and combusted inside a combustion enclosure. In this case thermal energy is exchanged by convection as cooler gasses enter the combustion enclosure mix with hotter gases and combust. Additionally convective thermal energy transfer also heats the combustion enclosure surfaces as gas passes thermal energy to the enclosure surfaces. Meanwhile the hot enclosure walls transfer thermal energy back to cooler gases entering the combustion chamber when hot surfaces emit thermal energy and gases flowing proximate to the hot surfaces are heated by the emitted radiation.
In conventional SOFC systems, a recuperator or gas counter flow heat exchanger, is disposed to receive hot gases exiting from the combustion chamber and to receive cool gases entering into the SOFC system in separate counter flow conduits separated by a common wall. Again convection and radiation are the dominant thermal energy transfer mechanisms as hot gases from the combustor heat conduit walls as they pass to an exit port and the conduit walls heat incoming air. In short the thermal energy exchange both inside the tail gas combustor and inside the recuperator is not efficient. The result is that conventional SOFC systems are notoriously difficult to control and often develop hot spots, e.g. in the combustion enclosures, that can damage the enclosure walls even burning through walls when a combustion enclosure wall gets too hot. Alternately when the temperature of the SOFC system is lowered, e.g. by modulating a fuel input flow rate, incomplete fuel processing results in carbon formation on anode surfaces which ultimately leads to decreased electrical output and eventual failure.
To better address hot and cold spots conventional SOFC systems often include a plurality of thermocouples or thermistors disposed at various system points to monitor temperature and adjust operation in order to avoid hot spots and prevent cold spots which lead to carbon formation on anode surfaces. However the temperature sensing and monitoring systems are costly and prone to failure due to the high operating temperatures of the SOFC systems (e.g. 350-1200° C.). Moreover the need to modulate fuel input as a safety measure to avoid damaging the SOFC system leads to inefficient and variable electrical power output. Thus there is a need in the art to avoid thermal gradients and eliminate hot spots in order to avoid damaging the SOFC system and in order to deliver more consistent electrical power output with improve power generation efficiency.
Conventional SOFC systems are generally fabricated from specialty materials in order to survive the effects of extended operation at high temperatures and the severely corrosive environment which continuously oxidizes metal surfaces sometimes to the point of failure. Other high temperature problems that have been addressed in conventional SOFC systems include the need to match or account for differences in the thermal coefficient of expansion of mating parts of dissimilar materials in order to avoid loosening between mating parts, cracking of ceramic elements or bending of metal elements, and the need to account for increased metal creep rates that occur at high temperature. In conventional SOFC systems these problems have been addressed by using specialty high temperature corrosion resistant nickel-chromium alloys such as Inconel or the like. However chromium leached into incoming cathode air can poison the cathode material layer, so materials that contain chromium are not desirable along any of the incoming air conduits or heaters if cathode poisoning is to be avoided. Thus while there is a need in the art to use corrosion or oxidation resistant high temperature metals alloys to fabricate SOFC hot zone elements many of these alloys contain chromium and there is a further need in the art to avoid contacting cathode air with chromium containing surfaces.
While some thermal energy is transferred between regions of conventional SOFC systems by thermal conduction, e.g. conducted across interconnected metal elements, the fact that hot and cold spots are still problematic in conventional SOFC systems suggests that thermal conduction is either too slow or insufficient to promote a uniform temperature across different regions of a conventional SOFC system. This is due in part to the need to use specialty metals for the high temperature corrosive environment which have less than desirable thermal conductivity properties. As an example, Inconel has a thermal conductivity ranging from 17-35 W/(m°K) over a temperature range of 150 to 875° C. as compared to copper which has a thermal conductivity approximately ranging from 370 W/(m°K) at 500° C. and 332 W/m° K at 1027° C. Thus copper has a thermal conductivity that is more than 10 times the thermal conductivity of Inconel, which is about 70% nickel. While copper provides increased thermal conductivity over high temperature metal alloys, mostly comprising nickel, which could improve temperature uniformity in SOFC systems, copper is readily oxidized in the SOFC environment and has thus far been avoided as an SOFC housing material.